Communication devices capable of implementing data communications and telephone calls wirelessly are well known in the art. In some cases, the frequency bands used for wireless communications by these communication devices overlap, even when the communication devices use different communication methods. When the frequency bands used for wireless communications by the communication devices overlap, the wireless communications of these devices interfere with each other, preventing the devices from achieving reliable wireless communications.
Generally, the frequency band appropriated for wireless communications is determined based on the communication method, while channels are designated based on differences in the center frequency of the frequency bands. However, the width of the appropriated frequency band varies according to the communication method, even when the channel number is the same. For simplification, the frequency bands used in wireless communications will be specified in the following description.
FIG. 6 outlines some common wireless communications standards for wireless LANs and digital cordless telephones that use the same frequency band. As shown in FIG. 6, the frequency band between 2.4 and 2.5 GHz is called the ISM (Industrial, Scientific, and Medical) band employed by industrial, scientific, and medical equipment that use radio waves for purposes other than communications.
For example, the ISM band is used for such medical equipment as laser scalpels, telecontrol devices and the like in factories, and cooking appliances such as microwaves. The frequency band from 2.4 to 2.497 GHz is used for the Bluetooth wireless communications protocol based on IEEE 802.15.1. The frequency band from 2.4 to 2.497 GHz is used in the wireless LAN standard based on IEEE 802.11b/g and is primarily employed in notebook computers. The frequency band from 2.4 to 2.4835 GHz is used in digital cordless telephones to implement wireless communications between like handsets and between the handsets and a base unit.
Since various devices perform wireless communications using the same frequency band in this way, there is always a chance that the wireless communications will interfere with each other.
To resolve this problem, Japanese unexamined patent application publication No. 2003-249973 discloses a communication controller that detects data for wireless channels used by each communication device and wireless channels that pose difficulties due to noise and the like in order to avoid the overlapping of frequency bands used in wireless communications by the communication devices. The communication controller periodically notifies each communication device of the data detected for the wireless channels. Based on this data, the communication devices select a wireless channel having a frequency band not currently used for wireless communications.
However, when the number of communication devices is large, the frequency band used by a communication device inevitably overlaps that used by another communication device, resulting in interference that prevents the communication devices from performing proper wireless communications.